1. Field of the Invention
The present invention generally relates to a piezoelectric actuator system with a position detection function and a method thereof, in particular, to a piezoelectric actuator system and a method thereof using the impedance detection to detect the position.
2. Description of Related Art
Piezoelectric actuator systems are actuator systems using voltages to achieve the effect of moving objects. In consideration of the production errors of the piezoelectric materials and the driving displacement errors, a conventional piezoelectric actuator system must be added with a positioning system. However, the conventional positioning system has a bulk volume, which hinders the miniaturization and simplification of the whole piezoelectric actuator system. Consequently, the conventional piezoelectric actuator system is inapplicable to small-sized precise devices such as lens modules of camera phones or pickup heads of thin-type optical disk drives.
FIG. 1A is a schematic side view of a conventional piezoelectric actuator system 10 in a stationary state. Referring to FIG. 1A, the conventional piezoelectric actuator system 10 includes an immovable base 12, a piezoelectric material 14, a drive shaft 16, and a movable object 18. The piezoelectric material 14 is fixed on the immovable base 12, and a length L of the piezoelectric material 14 varies according to the received voltage. That is to say, the piezoelectric material 14 deforms according to the received voltage. The drive shaft 16 is fixed on the piezoelectric material 14, and when the length L of the piezoelectric material 14 varies, the position of the drive shaft 16 changes accordingly. In FIG. 1A, the position of the drive shaft 16 is at x1. The movable object 18 is disposed on the drive shaft 16. When the drive shaft 16 moves slowly, the movable object 18 will move along with the drive shaft 16 because of the friction there between. However, when the drive shaft 16 moves fast, the movable object 18 stays at its original position based on Newton's First Law of Motion (law of inertia). In FIG. 1A, the position of the movable object 18 is at x2.
FIG. 1B is a schematic side view of the conventional piezoelectric actuator system 10 when a slowly increasing voltage is applied to the piezoelectric material 14. Referring to FIG. 1B, the slowly increasing voltage is applied to the piezoelectric material 14, so the length L of the piezoelectric material 14 becomes longer as compared with that when the piezoelectric actuator system 10 is in the stationary state. At this time, the drive shaft 16 moves due to the varying of the length L of the piezoelectric material 14. The movable object 18 moves along with the drive shaft 16 because of the friction there between, from x2 to x3.
FIG. 1C is a schematic side view of the conventional piezoelectric actuator system 10 when a rapidly dropping voltage is applied to the piezoelectric material 14. Referring to FIG. 1C, the rapidly dropping voltage is applied to the piezoelectric material 14, so the length L of the piezoelectric material 14 becomes shorter as compared with that under the conditions in FIG. 1B. The drive shaft 16 moves fast because the length L of the piezoelectric material 14 changes rapidly to the length obtained when the piezoelectric actuator system is in the stationary state. At this time, the position of the drive shaft 16 is at x1. The movable object 18 stays at its original position, that is, x3, based on Newton's First Law of Motion. As described above, by applying a slowly increasing voltage and a rapidly dropping voltage, the movable object 18 can be moved to a target position.
However, in consideration of the production errors of the piezoelectric materials and the driving displacement errors, the overall piezoelectric actuator system must be added with a positioning system. Referring to FIG. 2, a schematic side view of a piezoelectric actuator system 20 having a Hall sensor 21 is illustrated. The piezoelectric actuator system 20 includes the Hall sensor 21, an immovable base 22, a piezoelectric material 24, a drive shaft 26, a movable object 28, and a magnetic substance 25. The magnetic substance 25 is disposed on the movable object 28, and may be a magnet or a lodestone. The immovable base 22 includes a driving circuit 23. The driving circuit 23 is used for providing voltages to the piezoelectric material 24 and the Hall Sensor 21, and further detecting the voltage variation of the Hall sensor 21. The piezoelectric material 24 deforms under the influence of the voltage provided by the driving circuit 23.
The Hall sensor 21 senses a magnetic force emitted by the magnetic substance 25 disposed on the movable object 28. When the movable object 28 shifts, the magnetic force applied by the magnetic substance 25 on the Hall sensor 21 changes, and thus the induced voltage of the carrier of the Hall sensor 21 varies accordingly. The driving circuit 23 detects the voltage variations of the Hall sensor 21, and determines the position of the movable object 28 according to the voltage variations, thereby achieving the positioning and position detection functions.
However, in need of the Hall sensor 23 and the magnetic substance 25, the above piezoelectric actuator system 20 has a bulk volume, which goes against the miniaturization requirement.